I have previously identified the central and peripheral components of a parasympathetic neural circuit in birds that may be involved in the light-regulated control of blood flow in the choroid of the eye. This circuit consists of: retina - supra-chiasmatic nucleus (SCN) - medial subdivision of the nucleus of Edinger-Westphal (EWM) - choroidal neurons of the ciliary ganglion - choroidal blood vessels. The proposed studies will explore the role of this circuit in the visually-mediated control of choroidal blood flow and the functional significance of such control. Using laser Doppler velocimetry to monitor choroidal blood flow, I will examine: 1) the effect of activation of the central components of this circuit on choroidal blood flow; and 2) the possibility that light-elicited alterations in choroidal blood flow are mediated via this circuit. To explore the importance of this circuit for the health of retinal photoreceptors, I will examine the effects of SCN lesions on photoreceptor viability under different illumination and temperature conditions. Additional neural circuits also appear to be involved in the control of choroidal blood flow. To elucidate these circuits, retrograde and anterograde pathway tracing techniques will be used to: 1) identify the other regions of the central nervous system that give rise to input to EWM and may thereby influence choroidal blood flow; and 2) detail the central components of the other neural circuits that innervate choroidal blood vessels, including a sympathetic adrenergic input and a VIP-containing parasympathetic input from the sphenopalatine ganglion. These studies will provide the basis for future work directed at elucidating the role and importance of these circuits for the health of the retina. Since the choroid plays a vital role in the sustenance of the retina, neural control of the choroid may have an important influence on retinal functions. Disruption of this control may result in abnormal regulation of blood flow and disturbances in retinal function, possibly leading to a heightened susceptibility of retinal photoreceptors to the deleterious effects of light and eventually to photoreceptor degeneration. Since defective neural control of choroidal blood flow may underlie some of the degenerative diseases of human photoreceptors, further understanding of the neural control of the choroid may lead to better treatments for these diseases.